This invention relates to sound generation, and more particularly to a high power thermoacoustic sound generator having the unique capability of efficiently projecting sound into a body of water. The invention has utility in various applications such as geophysical exploration, anti-submarine warfare, environmental monitoring by acoustic tomography.
Conventional high power sound generators for underwater use utilize piezoceramic or magnetostrictive materials driven electrically by power derived from batteries. While these conventional generators are capable of delivering high power, they are subject to several drawbacks.
One drawback is the large space occupied by the conventional sound source, which includes not only the piezoceramic or magnetostrictive element, but also its associated parts. These associated parts may include one or more batteries, an oscillator, and amplifier, and a mechanical displacement multiplier.
Even highly efficient batteries, such as lithium batteries have limited energy and power densities. For example, the energy density for a LiSOCl.sub.2 battery is in the vicinity of 0.7 kilojoules per cubic centimeter (kJ/cm.sup.3), and its maximum power density is in the vicinity of 3 watts per cubic centimeter (W/cm.sup.3). While the oscillator does not ordinarily take up a significant amount of space, a power amplifier does. A power amplifier capable of producing an output of several kilowatts, even if designed for maximum power and minimum volume, may only produce about 2 W/cm.sup.3. Thus, from a power density standpoint, the amplifier may take up even more volume than the battery.
Another drawback, in the case of piezoceramic transducers, is the fact that they are inherently high impedance devices, requiring high voltage for their operation. Since battery voltage is limited by the number of cells, a power transformer is generally required. Furthermore, since the piezoceramic element is primarily capacitive, an impedance matching inductor is also generally necessary. The total power density for the transformer and inductor is in the vicinity of 3 W/cm.sup.3 /Khz. While a magnetostrictive transducer may not require a power transformer, since it is a high current device, an impedance-matching capacitor is usually required. The reactive elements needed to operate piezoceramic and magnetostrictive transducers, therefore, take up space in addition to that occupied by the batteries and power amplifiers.
Still another drawback is the fact that both piezoceramic and magnetostrictive transducers are limited in the amount of strain which they can produce. Displacement multiplying techniques, e.g. the so-called "flextensional" configuration described in U.S. Pat. No. 4,420,826, dated Dec. 13, 1983, are used to produce increased displacement. However, for such displacement multiplying techniques to be effective, the transducer must be backed by a medium more compliant than the water in which the transducer is immersed. Air is normally used to fill the inner volume of the transducer for this purpose. Unfortunately, the requirement for a compliant medium limits the depth of operation of the transducer since, at greater depths, the increased stiffness of the air reduces the displacement multiplication. Furthermore, for deep water operation, the transducer structure must be made strong enough to withstand the wide range of hydrostatic pressures encountered, or a pressure compensation system must be provided to keep the internal gas pressure close to the external hydrostatic pressure.
With conventional piezoceramic and magnetostrictive transducers, so long as the dimensions of the radiating surface of the transducer are smaller than the acoustic wavelength, the power output at a specific frequency is proportional to the square of the radiating surface area multiplied by the square of the displacement of the radiating surface. Therefore, if the transducer volume is constrained, it is necessary to produce large displacements to obtain high power output. But, to obtain large displacements, it is necessary to provide more or larger batteries, larger electronic components, and, in some cases, displacement multipliers. All of these elements take up substantial amounts of space. Therefore, it has not been possible to make small-sized, high power transducers for generating sound in water, using conventional piezoceramic and magnetostrictive transducers.
Another way to generate sound is by thermoacoustics. Thermoacoustic sound generation is accomplished by establishing a thermodynamic cycle in which an acoustic vibration itself provides the mass transport, and a stationary medium controls the heat transport. The process exploits the particular phase relationship between local pressure oscillations and local oscillations in the elemental volume in an acoustic wave. By appropriately positioning the second stationary medium in the acoustic flow and imposing a large temperature gradient on the second medium, heat can be transferred to and from the cycle with the phase required to generate high-amplitude oscillations. Although thermoacoustic sound generation is known, no practical way has heretofore been found to produce high power acoustic vibrations in bodies of water using thermoacoustic principles.